O-LevelChemistryOrganic Chemistry5070/22 · Oct/Nov 2014 · Q710 Marks

The alkanes are a homologous series of hydrocarbons.

(a)Give the name of another homologous series of hydrocarbons.[1]
(b)The graph below shows how the melting points of the first nine alkanes vary with the number of carbon atoms. [Figure showing a graph of melting point /°C vs number of carbon atoms for alkanes]
(b)Describe how the melting points of the alkanes with more than two carbon atoms vary as the number of carbon atoms increases.[2]
(c)Nonane is an alkane with nine carbon atoms. Give the molecular formula for nonane.[1]
(d)One mole of undecane, C11H24, is cracked to form a mixture containing one mole of ethene, one mole of propene and one mole of another hydrocarbon.
(d)(i)Construct the equation for this reaction.[1]
(d)(ii)Explain why oil companies crack the longer chain hydrocarbons.[2]
(e)Hydrogen cyanide, HCN, is manufactured by reacting methane with ammonia and oxygen. 2CH4 + 2NH3 + 3O2 → 2HCN + 6H2O
(e)(i)Calculate the mass of hydrogen cyanide that can be formed from 500g of methane if the percentage yield of hydrogen cyanide is 65%.[2]
(e)(ii)Hydrogen cyanide reacts with calcium hydroxide to form calcium cyanide and water. The formula of the cyanide ion is CN¯. Construct the equation for this reaction.[1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2014). The mark or mark type is shown in brackets.

[1] B7 (a) alkenes (1)
[1] B7 (b) melting points increase (1)
[1] B7 (b) increase in melting point from even number to odd number of carbon atoms is less than from odd to even number/the increase is less for some atoms than others/any reference to the regular zigzag nature of the increase (1)
[1] B7 (c) C9H20 (1)
[1] B7 (d) (i) C11H24 → C2H4 + C3H6 + C6H14 (1)
[1] B7 (d) (ii) (hydrocarbons with) longer chains not in high demand/more longer chains produced than used/shorter chains in more demand/fewer short chains produced than used (1)
[1] B7 (d) (ii) so (more) petrol/gasoline is made (1)
[1] B7 (d) (ii) to produce alkenes/to make ethane (1)
[1] B7 (e) (i) 16g methane → 27 g HCN (1)
[1] B7 (e) (i) 500 g methane → 500 × 27/16 × 65/100 = 548g (1) OR 500/16 = 31.25 mol methane (1) 31.25 × 27 × 65/100 = 548g (1)
[1] B7 (e) (ii) Ca(OH)2 + 2HCN → Ca(CN)2 + 2H2O (1)

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